Heat exchanger apparatus



'its lengthwise traverse of the shell.

'U-tubes or by a floating return header.

flow results in much lower thermal efliciency.

United States Patent 3,242,983 HEAT EXCHAN GER APPARATUS Noel H. deNevers, Salt Lake City, Utah, assignor to Chevron Research Company, acorporation of Delaware Filed Jan. 14, 1965, Ser. No. 426,481

3 Claims. (Cl. 165-159) This is a continuation-in-part application ofHeat Exchanger Apparatus, N. H. de Nevers, filed December 13, 1961,Serial No. 159,063, now abandoned.

This invention relates to an improved type of shell and tube heatexchanger.

The invention solves certain prior art problems that exist to a greaterextent in large heat exchangers, i.e., those exchangers having shellslonger than about 5 to feet than in smaller heat exchangers of the sametype.

In small shell and tube heat exchangers, one fluid passes through manytubes while another fluid flows through a cylindrical shell surroundingthe tubes. The tubes are fixed (by rolling or welding) to tube sheetswhich form the end closures of the cylindrical shell. These end closuresprevent any mixing of the two fluids. Baflies generally are locatedtransversely in the shell to force the shell fluid to flow back andforth across the tubes during In these small exchangers a very closeapproach to true countercurrent flow is achieved. True countercurrentflow is desirable because it gives the greatest heat transfer efficiency(maximum heat transferred per unit area of heat transfer surface).

In large heat exchangers (shells longer than about 5 to 10 feet), theaforesaid method of construction is not feasible. The difference in athermal expansion of the shell and tube is conducive to the rupture ofthe tube sheet or the tubes. The flexibility of the material is adequateto take up this differential expansion in the small units, but not inthe larger units, for which some other means must be found to preventtube and/ or sheet rupture from thermally-induced stresses.

The customary solution for this thermal expansion problem has been toprovide for the fluid in the tubes to be returned to the inlet end ofthe exchanger either by However, this solution of the thermal expansionproblem is achieved at the cost of a significant reduction in thermalefliciency of the exchanger. This reduction is a result of passing thetube side fluid countercurrent to the shell side fluid in only half ofthe exchanger and passing it cocurrent to the shell side fluid in theother half of the exchanger. Cocurrent As a result to obtain adequateheat transfer, the designer must alter design variables of the exchangerto compensate for the lower heat transfer efiiciency as for example byincreasing the length of the exchanger. With such changes, however,there is a corresponding increase in fabrication and repair costs of theexchanger.

The object of the invention is the provision of an improved multipassheat exchanger and method of fabrication in which space within theexchanger is economically allocated to maximize the heat transferbetween the shell and tube side fluids and simultaneously reduce thesize of the shell cover, headers, tubes, etc. heretofore required toprovide such transfer.

Conventional two-tube-pass one-shell-pass heat exchangers normally aredesigned with an efliciency factor of as low as 80% (i.e., having athermal efficiency only 80% of that of a true countercurrent exchanger).The exchanger of the present invention has a thermal efiivciency muchmore closely approaching that of a true "ice the design reduces the sizeof the exchanger over those of prior art exchangers that provide similarinlet and outlet temperatures for the shell side and tube side fluids.

The invention will best be understood, and. further objects andadvantages thereof will be apparent, from the following description whenread in connection with the accompanying drawing in which:

FIGURE 1 is a sectional elevation view illustrating a conventionaltwo-tube-pass one-shell-pass heat exchanger of the aforesaid type,namely one in which tubes for one fluid medium are longitudinallydisposed within the shell, and baffles are provided for elongating thepath of the flow of the other fluid through the apparatus;

FIGURE 2 is a sectional elevation view illustrating an embodiment of thepresent invention, also comprising tubes for one fluid longitudinallydisposed within the shell and battles for elongating the path of theflow of the other fluid through the apparatus;

FIGURE 3 is a transverse sectional view taken along the lines 3-3 inFIGURE 2.

In accordance with the present invention, there is provided a compactmultipass heat exchanger for maximizing heat transfer between shell sideand tube side fluids in which the tube side fluid passes through amultiplicity of small diameter tubes in countercurrent flow with thepath of the shell side fluid, and returning therethr-ough by a singlelarge conduit in concurrent flow with the shell side fluid. The smalltubes are transversely located across the exchanger to substantiallyfill the interior of the shell. In that manner, the heat transfer areaof the tubes in the countercurrent flow direction is increased over thatof conventional exchangers. In addition, the economical spacing of thesmall tubes and return conduit within the shell, provide an over-alldecrease in the size of the exchanger in that the transverse spacingrequired to provide mechanical support for the single return conduitoccurs only over one radial section of the exchanger, not over theentire interior region, as in the conventional exchangers.

Further in accordance with the present invention, it will be understoodthat the invention as set forth in the foregoing paragraph may bemodified by reversing operation thereof, for example by passing saidsecond fluid first through said conduit and thence through said tubes,and by reversing the flow of said first, or shell, fluid through saidapparatus to maintain tube fluid flow countercurrent to said shell fluidflow and to maintain the flow of tube fluid in said conduit in cocurrentflow with the shell fluid.

The invention is applicable to shell and tube heat exchanger apparatushaving at least two tube passes and at least one shell pass. It is ofthe essence of the invention to provide, in any such type of apparatus,for passing tube fluid longitudinally through the apparatuscountercurrent to the shell fluid in at least eight times as manyuniformly disposed fluid streams as there are tube fluid streams flowingcocurrent to the shell fluid. Preferably, there will be 25 to 600, morepreferably to 500 uniformly disposed tube fluid streams flowing throughthe apparatus countercurrent to the shell fluid. These streams will bedisposed within the apparatus throughout the interior of the shellincluding the space transverse to the axis of symmetry and the shell. Anexemplary conventional two-tube-pass one-shell-pass shell and tube heatexchanger might contain 500 tubes, each onehalf inch in diameter, inwhich tube fluid would flow countercurrent in 250 tubes and cocurrent in250 tubes. An exemplary apparatus of the present invention, which wouldbe used in lieu of said conventional apparatus, would comprise 450transversely disposed tubes, each onehalf inch in diameter, and one tubeabout six inches in diameter. The tube fluid in the latter apparatuswould flow countercurrent to the shell fluid in the 450 one-halfinchtubes and cocurrent in the single six-inch tube. The large tube orconduit in the apparatus of the present invention is preferablylongitudinally disposed within the heat exchanger shell toward theperiphery thereof and has sufficient area to pass the tube side fluidafter the latter exists from the small tubes without change in flowpattern. The large conduit also reduces the transverse dimensions of theexchanger thereby reducing fabrication costs in that the spacingrequired for mechanical support of the single return conduit occurs onlyover one radial section of the exchanger, not over the entire centralregion as in conventional exchangers. It will be understood that morethan one large tube or conduit may be used, so long as the aforesaidconditions are fulfilled.

It has been found that a higher mean temperature difference may beobtained with the aforesaid apparatus, enabling more heat transfer totake place than in a conventional apparatus with the same heat transferarea, i.e., in an exchanger having tubes of equal size in both thecocurrent and countercurrent flow directions.

It has been found that the apparatus accomplishes a closer approach totrue countercurrent flow by providing for tube fluid flowscountercurrent to the shell current in many small tubes and cocurrent tothe shell fluid in one large conduit. The resulting heat transfer perunit area of exchanger provides a higher thermal efficiency for theapparatus enabling a reduction in the all-over dimensions of theexchanger over those of conventional exchangers.

Referring now to FIGURE 1, there shown is a sectional elevation view ofa conventional type of shell and tube heat exchanger. Tubes 1, forcarrying a first fluid medium are longitudinally disposed in elongatedpressure vessel or shell 2, as shown. Tube sheet 3 and separator 4divide one end of shell 2 into an inlet chamber 5 and an outlet chamber6. A first fluid medium is passed through inlet 7 into inlet chamber 5,thence through tubes 1 to outlet chamber 6, and thence through outlet 8.A second fluid medium is passed through inlet 9 into shell 2, thencelongitudinally through shell 2 by an elongated pathway created bybaffles 10, and thence through outlet 11. In such a heat exchanger,either fluid may be at a higher temperature and will convey heat to theother fluid through the walls of tubes 1, acting as intermediary heatexchange materials. It will be noted that in this conventional type ofheat exchanger expansion of the tubes introduces no stress on the shell.

Referring now to FIGURE 2, there shown is a sectional elevation viewillustrating an embodiment of the present invention, comprising smalltubes distributed throughout the interior of the shell including thespace transverse thereto, and a large conduit longitudinally disposedWithin the shell for passage of tube fluid through the apparatus andbaffles for elongating the path for the flow of the shell fluid throughthe apparatus. Tubes 21, for carrying a first fluid medium are disposedin elongated pressure vessel or shell 22 as shown. Tube sheet 23 andseparator 24 divide one end of shell 22 into an inlet chamber 25 and anoutlet chamber 26. Tube sheet 27 and cover 35 form a floating head fluidtransfer chamber 28, as shown.

A large conduit 29 is disposed in shell 22, as shown, coextensive withtubes 21. It has a cross-sectional area that may be substantially lessthan the sum of the crosssectional areas of tubes 21.

There is an economic balance to be struck in the design of the size ofthe conduit 29 between low pressure drop through the conduit and minimumsurface area contained in the conduit. It can be shown that the minimumcrosssectional area of the conduit 29 need be only small large 21 tomaintain an equal pressure drop in each direction of travel of the tubeside fluid, where dsman is the diameter of tubes 21 and dlarge is thediameter of conduit 29 Of course the linear velocity of the fluid inconduit 29 relates to velocity of the fluid in tubes 21 in the samemanner as that which exists for their respective areas. Consequently, itis desirable that 'care be taken in the selection of the respectivesizes of these members to maintain a uniform flow pattern of the tubeside fluid under operating conditions.

A first fluid medium is passed through inlet 30 into inlet chamber 25,thence through tubes 21 to fluid transfer chamber 28, thence throughlarge conduit 29 into outlet chamber 26, and thence through outlet 31. Asecond fluid medium is passed through inlet 32 into shell 22, thencelongitudinally through shell 22 by an elongated pathway created bybaflies 33, and thence through outlet 34. Either the tube fluid or theshell fluid may be at a higher temperature and will convey heat to theother fluid through the Walls of tubes 21, acting as intermediary heatexchange materials. It will be seen that, in operation of the embodimentshown, a number of separate tube fluid streams, flowing therethroughcountercurrent to the flow of shell fluid at locations radiallydistributed throughout the interior of the shell can be madesubstantially greater than the number of fluid streams flowing throughthe apparatus cocurrent to the shell fluid. Therefore more heat transfercan be caused to take place in the apparatus with the same heat transferarea than in the case with conventional shell and tube heat exchangerapparatus. In addition, the one fluid stream flowing cocurrent with theshell fluid can be directed therealong by a single conduit thateconomically utilizes space within the exchanger.

Referring now to FIGURE 3, there shown is a transverse sectional viewtaken along the lines. 33 in FIG- URE 2. Tubes 21 are shown disposedthroughout the interior of the shell in spaced locations for passing thetube fluid through the apparatus in countercurrent flow to the shellfluid. The centers of these locations may define a series of circleshaving a common origin located at the axis of symmetry of the exchanger,but preferably the tube arrangement involves positioning the tubesacross the entire interior of the shell in either (1) equilateral, (2)staggered square, or (3) in line square tube arrays as conventionallyunderstood in the art. The spacing between each tube in any of theabove-mentioned arrays is preferably equal to prevent hot spots fromdeveloping during operation of the exchanger and to equalize therelative pressure drop per unit section of the exchanger.

Large conduit or tube 29 is shown disposed within shell 22 for tubefluid flow through the apparatus in cocurrent flow with the shell fluid.The conduit 29 is preferably disposed close to the periphery of shell 22as shown and attaches at its ends to the tube sheet, one of whichpermanently attaches to the sidewall of the shell 22. The strength ofthe tube sheets, of course, is directly related to the type of materialused and inversely related to the number of openings formed therein toreceive the tubes 21 and the conduit 29. Consequently by using only asingle conduit, the amount of material required for support can besubstantially reduced, if desired. The design thus provides that theconduit need be supported in only one radial location of the tube sheet,not across their entire surfaces as in conventional exchangers. Thesaving in space can be used to reduce the size of the exchanger orincrease the number of tubes passing the tube fluid in countercurrentflow with the shell side fluid. However, the flow capacity of theconduit 29 must be designed so as to return the tube fluid through theexchanger without a change in flow pattern.

It will be seen that the apparatus of the present invention inherentlyresults in distributing the total cross-sectional area of countercurrentflow tubes 21 over a much greater portion of the cross-sectional area ofshell 22 than in conventional shell and tube heat exchanger apparatus.Consequently the apparatus of the present invention accomplishes greaterheat transfer than in conventional similar apparatus having the sametotal heat transfer area so that the heat transfer efficiency thereforeis superior. Furthermore, there is provided a constuction that allows areduction in the size of the exchanger but with no correspondingreduction in performance.

In the design and fabrication of the exchanger in accordance with thisinvention, there will necessarily be involved a re-examination of theheat balance equations heretofore used to describe the operation ofmultipass heat exchangers. In previous designs the cross-sectionaldimensions of the tubes used to convey the tube side fluid have alwaysbeen assumed to be equal. See for example, pages 340 et seq., Principlesof Engineering Heat Transfer, Warren H. Geidt, D. Van Nostrand, 1957. Inthe optimization of the exchanger in accordance with the invention,there are involved tubes of unequal dimensions. Therefore the solutionof these heat balance equations will be of increased complexity, and forthis purpose the assistance of acomputer may be desirable.

In one modified design approachcalled the logarithmic-mean temperaturecorrection -approaohit can be shown for a single-shell-pass,two-tube-pass exchanger with unequal tube sizes in the two tube passes,that the temperature of the shell side fluid, t at any location alongthe length of the exchanger can be expressed by d tt (a-l-b) U ab dac dz0., C, 0

where: U is the average over-all conductance of the entire exchanger,

In the above equation, it is assumed that the average over-allcoeflicient, U, is constant throughout the exchangers, the fluidcapacity rates are constant, and the fluid mixing is suflicient to makethe temperature uniform across any cross section of the exchan er.

In order to achieve a workable methematical model of the heat transferusing the above equation, the above equation is first solved; then thesolution is differentiated to relate the heat transfer in the exchangerto the temperature of the shell side fluid, t along the length thereof,by considering the rate of change of t with the length of the exchanger.

Mean temperature difference between the tube side and shell side fluids,Ar is then determined in terms of the inlet and outlet temperatures ofthe fluids using Newtons basic cooling laws in conjunction with thelast-mentioned equation, i.e., the equation describing the rate ofchange of the temperature of the shell side fluid t with exchangerlengths.

If desired, nomographs can be prepared using a dimensionless correctionfactor where:

Ar is the mean temperature difference between the tube and shell fluids,and

At is the log mean temperature difference between these fluidscalculated for countercurrent flow.

In calculating optimum cross-sectional dimensions for the exchanger, itwill be necessary in some cases to assume a series of differenttemperature difference between 6 the fluids before optimum dimensionscan be found as is presently practiced in the art.

Another approach which eliminates the trial-and-error calculations notedabove is the effectiveness-number of heat transfer units approach. It isbased on the fact that:

(1) the average over-all conductance of the entire exchanger,

(2) the heat transfer area of the exchanger,

(3) the fluid capacity rate of the shell side fluid, and

(4) the fluid capacity rate of the tube side fluid,

are functions of the terminal temperatures of the fluids. This approach,however, again must take into consideration the change in tubedimensions, and basically involves optimizing the cross-sectionaldimensions by comparing the actual heat transfer rate of the exchangerwith the thermodynamically limited maximum heat transfer rate that couldpossibly be realized.

For fabrication purposes, it may also be desirable to use a plurality oflarge tubes or conduits 29; but this is not a preferred form of theapparatus of the present invention and in any event the invention wouldrequire at least eight and preferably 25600 and still more preferably-50 times as many small tubes as large conduits.

It will become more apparent to those skilled in the art that variousmodifications and variations may be made in the apparatus of the presentinvention as described above. Consequently all modifications that arewithin the spirit of the invention are intended to be included withinthe scope of the appended claims.

I claim:

1. In shell and tube heat exchanger apparatus comprising an elongatedheat exchanger shell having an axis and a sidewall, a fluid inlet for afirst fluid at one end thereof and a fluid outlet for said first fluidat the other end thereof, the improvement which comprises a bundle ofelongated tubes longitudinally disposed in said shell for passage of asecond fluid within said tubes in two passes through said shell, inletmeans at one end of said shell for conveying said second fluid into saidtubes without physical contact with said first fluid and outlet means atthe other end of said shell for conveying said second fluid from saidtubes without physical contact with said first fluid, said bundleincluding the combination of a multiplicity of tubes of relatively smallcircular cross section for said first pass of said second fluid throughsaid apparatus in countercurrent flow with the pass of said first fluidthrough said apparatus and a single large conduit of constant circularcross-section for said second pass of said second fluid through saidapparatus in cocurrent flow with the pass of said first fluid throughsaid apparatus thereby forming a compact heat exchanger apparatus havingmaximum thermal efficiency by decreasing the relative heat transfer ofsaid first and second fluids in said cocurrent flow direction andincreasing the relative heat transfer in said countercurrent flowdirection, said large conduit having a minimum cross-sectional areaequal to:

fi X Asmall lar e where dsman is the diameter of each of saidmultiplicity of tubes, dlarge is the diameter of said large conduit andAsman is the total cross-sectional area of said multiplicity of tubes,said multiplicity of tubes being transversely and symmetrically disposedwithin said exchanger across the interior of said shell unoccupied bysaid large conduit to provide maximum heat transfer area between saidfirst and second fluids in the countercurrent flow direction, said shellfluid forming a moving column of fluid immediately adjacent to theexterior surface of said large conduit as said first fluid travelsbetween said fluid inlet and said fluid outlet therefor.

2. Heat exchanger apparatus comprising an elongated shell having an axisand a sidewall, a plurality of circular cross-sectional tubestransversely disposed within said shell across the interior thereofbetween said axis and said sidewall for passing tube fluid through saidapparatus generally countercurrent to shell fluid at a desiredvolumetric rate, a single conduit of circular cross-sectiondisposedwithin said shell coextensive with said plurality of tubes, saidconduit having a minimum circular crosssectional area equal to:

large where d is the diameter of each of said multiplicity of tubes, dis the diameter of said large conduit and Asman is the totalcross-sectional area of said multiplicity of tubes to return said tubefluid through said apparatus at said rate in a return pass cocurrent tosaid shell fluid thereby to maximize thermal efliciency of saidapparatus by decreasing relative heat transfer of said tube fluid andsaid shell fluid in said cocurrent flow direction and increasing heattransfer in said countercurrent flow direction, means for introducingsaid tube fluid into said tubes, means for directing said tube fluidfrom said tubes through said conduit, means for withdrawing said tubefluid from said conduit, and means for passing said shell fluid in asinusoidal path through said apparatus countercurrent to said tube fluidin said tubes and cocurrent to said tube fluid in said conduit, saidshell fluid forming a moving column of fluid in the region immediatelyadjacent'to the exterior surface of said single conduit as said shellfluid passes through said apparatus.

3. Heat exchanger apparatus comprising an elongated shell, a singlelarge conduit of circular cross-section longitudinally disposed withinsaid shell for passing tube fluid through said apparatus generallycocurrent with shell fluid at a desired volumetric rate, a plurality oftubes longitudinally transversely disposed within said shell in radiallyspaced locations across the entire interior of said shell, saidplurality of tubes having a total cross-sectional area equal to:

small large where A is the cross-sectional area of said single largeconduit, dsman is the diameter of each of said multiplicity of tubes andd is the diameter of said large conduit to return said tube fluidthrough said apparatus at said rate in a return pass countercurrent tosaid shell fluid thereby decreasing relative heat transfer of said tubefluid and said shell fluid in said cocurrent flow direction andincreasing heat transfer in said countercurrent flow direction tomaximize thermal efliciency of said apparatus, means for introducingsaid tube fluid into said conduit, means for directing said tube fluidfrom said conduit through said tubes, means for withdrawing said tubefluid from said tubes, and means for passing said shell fluid in asinusoidal path through said apparatus countercurrent to said tube fluidin said tubes and cocurrent to said tube fluid in said conduit.

References Cited by the Examiner UNITED STATES PATENTS 167,182 7/1875Mallory l,-l46 1,497,491 6/1924 Elliott -146 X 1,856,771 5/1932 Loefller165-155 X 2,468,903 5/1949 Villiger 165155 2,942,855 6/1960 Wellensiek165159 X FOREIGN PATENTS 596,021 7/ 1959 Italy.

267,189 6/ 1950 Switzerland.

KENNETH W. SPRAGUE, Primary Examiner.

FREDERICK L. MATTESON, JR., Examiner.

1. IN SHELL AND TUBE HEAT EXCHANGER APPARATUS COMPRISING AN ELONGATEDHEAT EXCHANGER SHELL HAVING AN AXIS AND A SIDEWALL, A FLUID INLET FOR AFIRST FLUID AT ONE END THEREOF AND A FLUID OUTLET FOR SAID FIRST FLUIDAT THE OTHER END THEREOF, THE IMPROVEMENT WHICH COMPRISES A BUNDLE OFELONGATED TUBES LONGITUDINALLY DISPOSED IN SAID SHELL FOR PASSAGE OF ASECOND FLUID WITHIN SAID TUBES IN TWO PASSES THROUGH SAID SHELL, INLETMEANS AT ONE END OF SAID SHELL FOR CONVEYING SAID SECOND FLUID INTO SAIDTUBES WITHOUT PHISICAL CONTACT WITH SAID FIRST FLUID AND OUTLET MEANS ATTHE OTHER END OF SAID SHELL FOR CONVEYING SAID SECOND FLUID FROM SAIDTUBES WITHOUT PHYSICAL CONTACT WITH SAID FIRST FLUID, SAID BUNDLEINCLUDING THE COMBINATION OF A MULTIPLICITY OF TUBES OF RELATIVELY SMALLCIRCULAR CROSS SECTION FOR SAID FIRST PASSES OF SAID SECOND FLUIDTHROUGH SAID APPARATUS IN COUNTERCURRENT FLOW WITH THE PASS OF SAIDFIRST